Making fused thorium carbide-tungsten cathodes for electron guns

ABSTRACT

The surface energy of a liquid phase of thorium compounds or alloys is used in fabricating cathodes by immersing the lower end of an inclined refractory metal tube into a powder mixture of elemental thorium and tungsten carbide, heating to a temperature above the melting point of the mixture so that the liquid mixture flows up the inside of the tube by capillary attraction, cooling and cutting the tube into sections of length for cathode bodies. A convex hemispherical emissive surface for the thermionic cathode is formed by electron beam melting the end of the thin rod of a thorium compound or alloy.

United States Patent [72] Inventor Homer H.Glascock, l r.

Scotia, N.Y. (21] Appl. No. 832.758 [22) Filed June 12, 1969 [4S] Patented June 29, 1971 [73] Assignee General Electric Company [54] MAKING FUSED THORIUM CARBIDE-TUNGSTEN CATHODES FOR ELECTRON GUNS 8 Claims, 6 Drawing Figs.

[52] U.S.Cl 264/25, 29/1827, 29/1828, 106/43, 264/61,264/267, 264/332, 313/346 [51] lnt.Cl ..1l0lj 29/04, H01 j 29/48 [50] Field of Search 264/25, 61, 157, 267, 332, 349; 313/31 1, 346;106/43; 29/182.7, 182.8

[56] References Cited UNITED STATES PATENTS 3,160,780 12/1964 Coppola 313/346 3,258,636 6/1966 Affleck et a1. 106/43 3,269,804 8/1966 Affleck et a1. 313/346 Primary Examiner-.lulius Frome Assistant Examiner.lohn H. Miller AltameysPaul A. Frank, John F. Ahern, Julius .1.

Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman 1 ABSTRACT: The surface energy of a liquid phase of thorium compounds or alloys is used in fabricating cathodes by immersing the lower end of an inclined refractory metal tube into a powder mixture of elemental thorium and tungsten carbide, heating to a temperature above the melting point of the mixture so that the liquid mixt'ure flows up the inside of the tube by capillary attraction, cooling and cutting the tube into sections of length for cathode bodies. A convex hemispherical emissive surface for the thermionic cathode is formed by electron beam melting the end of the thin rod of a thorium compound or alloy.

PATENTEU JUN29 I97l FIG. 2

FIG. 4

lNVE/VTOR HOMER H. G'LASCOCK, JR, by m HIS ATTORNEY MAKING FUSED TIIIORIUM CARBIDETUNGSTEN CATHODES FOR ELECTRON GUNS My invention relates to thorium compound cathodes and, in particular, to new and improved methods of making fused thorium compound cathodes and the resultant cathodes.

Thermionic cathodes composed of an aggregate of particles of thorium carbide, or thorium carbide and tungsten, have been used in a variety of electron gun applications. These cathodes conventionally are fabricated by compressing mixed thorium and carbon, or thorium and tungsten carbide powder into a refractory metal cup at a pressure of several tons per square inch. With subsequent heating in vacuum to operating temperatures, a chemical reaction converts the mixture to a sintered thorium carbide or thorium carbide plus tungsten.

Thorium carbide cathodes possess a higher work function than do other conventional cathodes, such as barium oxide or barium dispenser cathodes, and also operate at much higher temperatures. As a consequence, thorium carbide cathodes consume more power and exhibit greater emission cooling for the same level of emission than do barium cathodes. On the other hand, thorium carbides do possess the outstanding ability to yield high emission current density in a rather poor vacuum for long periods of time. Moreover, thorium carbide has a rather high electrical conductivity so that cathodes formed from it suffer from none of the problems associated with insulating or semiconducting emitters.

Pressed and sintered thorium carbide plus tungsten cathodes have been used in large numbers, yielding several amperes per square centimeter for many thousands of hours. After long usage, it has been found that the shrinkage of the porous emission mixture at operating temperature moves the front surface of the cathode causing changes in the electron optical properties of the tube utilizing such a cathode. A sig nificant transmission change is observed from a cathode surface withdrawal of as little as 0.0001 inch.

It is a primary object of my invention to provide a new and improved fused thorium compound cathode and methods and apparatus for making such fused cathodes.

It is another object of my invention to provide a new and improved fused thorium compound cathode and methods and apparatus for making such cathodes which permit maximizing the current density of the cathode in a small beam spot.

It is another object of my invention to provide new and improved methods for manufacturing thorium compound cathodes which permit the formation of a fused cathode composed of material having a desired composition.

In its broadest aspect, my invention consists in making a fused thorium compound cathode by melting a mixture of the desired thorium and other metal powders, suspending a tube of a refractory metal in the molten mixture so that its lower end is immersed in the molten mixture, and allowing the'mixture to ascend the tube by capillary attraction, the resultant filled tube, upon cooling, being divided into cathode bodies of desired length. Another feature of my invention consists in bombarding such a cathode body with an electron beam to produce melting of the end of the body and thereby form a smooth hemispherical surface which when used in an electron optical device maximizes the density of the current emitted by the cathode in a small beam spot.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 illustrates a starting step in my method of making a thorium compound cathode;

FIG. 2 illustrates an intermediate step in the making of such a cathode;

FIG. 3 illustrates a variation in my improved method of making a thorium compound cathode;

FIG. 4 is a schematic drawing of the principal elements of an electron discharge device employing my improved thorium compound cathode;

FIG. 5 illustrates a modification of my method and apparatus for making a thorium compound cathode; and

FIG. 6 illustrates a modified thorium compound cathode embodying additional features of my invention.

In my capillary method of cathode fabrication illustrated in FIG. 1, a sample of a chosen emission mixture 1 is placed in a container or boat 2 and a refractory metal tube 3 is inserted in the mixture. The mixture 1 may comprise any chosen mixture of emission powders, for example, thorium carbide powder or a mixture of thorium and tungsten carbide powders. The boat or container 2 may comprise any suitable refractory metal, such as, for example, a tungsten container. Refractory metal tube 3 may, likewise, comprise any satisfactory refractory metal such as, for example, a tube formed of an alloy of tungsten and rhenium, a conventional alloy consisting of approximately 75 percent tungsten and 25 percent rhenium. Tube 3 is supported by conventional means (not shown) so that its lower end remains immersed in the powders during subsequent manufacture of the thorium compound cathode.

After refractory metal tube 3 is supported in position in container 2, the entire assembly is heated until the emission mixture melts. Thus, for example, the entire assembly may be placed in a refractory container and heated by radiofrequency currents in vacuum until the emission mixture melts. Upon melting, I employ the surface energy of the liquid phase of the thorium compound to form a dense, fused cathode structure. Thus, upon melting of the powder mixture, capillary attraction draws the liquid up into the refractory metal tube 3. The height, h, to which the liquid will rise in metal tube 3 may be expressed approximately by the equation where:

S is the liquid-vapor surface tension 6 is the angle of contact of the liquid with the tube p is the liquid density r is the inside tube radius and g the local gravitational acceleration term.

In one manufacturing process in which the powder comprises thorium'and tungsten carbide and the internal diameter of the tube 3 was 0.032 inch, a molten mixture rose to a height of 1.7 inches. I found that this was suffieient length to make 28 cathode bodies of the length of a typical thorium dispenser cathode.

FIG. 2 illustrates the assembly of FIG. 1 after the powder mixture has been melted and the surface energy of the liquid phase of the melted powder has caused it to rise by capillary attraction in tube 3.

FIG. 3 illustrates how the length of the filled tube can be increased by inclining the tube at an angle D with the vertical. The length of the filling in the tube is then expressed by l=h sin I After the tube 3 has been filled with liquid emission mixture, it is allowed to cool. It is then cut into cathodebodies of the desired length, typically 0i'050 inch long for 0.030- to 0.040 inh'e'xt'erhal diameter of tube 3.

FIG. 4 is a schematic drawing of the essential portions of an electron discharge device embodying the cathode of my invention. Inthis structure, the cathode body 5 formed by utilizing capillary attraction to fill the refractory metal tube with a liquid emission mixture, is supported by a pair of legs 6, 7 which provide externally accessible terminals for the cathode body. Opposed to the emissive surface of cathode 5 is an anode 6 having a narrowed neck portion 8, the outer end of which is spaced a desired distance from the surface of cathode 9. Anode 8 is surrounded by an insulating cylinder 10 of any suitable material such as, for example, alumina, which in turn is supported in a rigid supporting member 11 for maintaining the surface of anode 9 in fixed spaced relation with respect to the opposed surface of cathode 5.

in operating an electron discharge device embodying a cathode of my invention, l have found that a cathode formed by capillary attraction is superior in a number of respects to a cathode formed by packing emissive powders in a supporting cylinder. In particular, it is found to be more resistant to hydrolysis and, likewise, is mechanically stronger than prior cathodes and hence easier to handle. In addition, the cost of producing the cathode by the capillary attraction method of my invention is much less than the cost of producing a cathode by packing powders in a refractory tube and thereafter sinter ing the packed powders.

I have found also that the cathode of my invention possesses a more mechanically stable emitting surface and has a lower evaporation rate than does the packed powder cathode. The mechanical stability of the cathode emission surface is particularly desirable for use in electron tubes where it is desired to avoid changes in the electron optical properties of the tubes. In operating a cathode comprising thorium carbide and tungsten made in accordance with my method at an operating temperature of 2050 K, the cathode yielded 3 amperes per square centimeter and was operated at this temperature for 1000 hours with no change in emission capability.

FIG. illustrates a modification of the apparatus and method used in manufacturing cathodes in accordance with my invention which is particularly useful where an emission mixture is used which does not possess a fixed melting point. Thus, in some circumstances, where a noncongruently melting emission mixture is used, a near eutectic liquid may be drawn off where the refractory metal tube is immersed in the emission mixture powder prior to melting. In such a case, the resulting tube fill has quite different composition from that of the original powder mixture.

In the apparatus of FIG. 5, the mixture of powders 1 is placed in a boat or container 15 and suspended by means of legs 16 from the inturned lower edges 17 of a U-shaped support 18. Contained within support 18 are a plurality of tungsten guides 19 having one end attached to the wall of support 18 and their other end 20 partially encircling tube 3. l, likewise, provide a fusible support 21, the ends of which are attached to the walls of support 18 and which is attached to tube 23 to maintain it in a position where it is supported above the top of the mixture of powders i.

In forming a cathode utilizing the apparatus of FIG. 5, supporting strip 21 is formed of a material which has a melting point higher than the melting point of the powders of mixture 1. Accordingly, after the powders in boat 15 are melted, the entire assembly is heated to slightly above the melting point of strip 21. Tube 3 then drops into a completely molten emission mixture which flows up the tube and invariably yields the desired composition of the material for the cathode body.

When the powders-which are to form the cathode material are an equal molar mixture of thorium and tungsten carbide, for example, I have found that fusible strip 21 may be formed of niobium. Accordingly, when the assembly is heated to a temperature slightly above 2468 C., the melting point of niobium, tube 3 will drop into the molten mixture. To assist in causing the tube to descend into boat 1 when strip 21 fuses, a weight 22 formed of a suitable high-melting material, such as, for example, molybdenum, may be placed on top of the tube to assure that the tube drops into the molten mixture.

For highest quality cathodes, an equal molar mixture of I thorium carbide and tungsten is desired since this mixture results in a fixed interlocking tungsten dendritic structure with the interstices filled with the thorium carbide-tungsten eutectic COEIPOSIIIOI]. Use of the apparatus of FIG. 5 is particularly desirable in this case since the eutectic composition of the thorium carbide-tungsten alloy system is on the thorium-carbide-rich side of the phase diagram. If the refractory metal tube is immersed in the emission powder rise prior to the melting of the mixture powder, there is a tendency to draw off the eutectic composition into the tube leaving tungsten in the boat. However, by supporting the refractory tube in the manner illustrated in FIG. 5, the tube will drop upon the melting of the fusible strip 21 into a completely molten emission mixture which then flows up the tube and invariably yields the desired tungsten dendritic structure when quenched which results in cathodes with very mechanically stable surfaces.

In many electron optical applications, a recurring problem is maximizing the current density of a cathode in a small beam spot. Since maximum spot brightness is directly limited by the cathode brightness, it is desirable to maximize cathode emission density by effectively focusing the cathode emission into a beam or small spot corresponding to a virtual point source. Thus, when using my cathodes in certain electron optical applications, instead of having a planar surface for the emissive end of the cathode, it is desirable to have a surface which is smooth and uniform and which, upon the application ofa high electric field, permits focusing of the cathode emission as from a small spot or point source. Accordingly, for such applications, the tip of a cathode body formed by the methods illustrated in FIGS. 1 and 5 is melted by electron beam bombardment so that surface tension forms it into a spherical surface after which it is allowed to solidify. Such a structure is illustrated in FIG. 6 in which the cathode body 5, after being mounted in heater legs 6, 7, is bombarded by an electron beam by a conventional means (not shown) so that the tip 30 melts and surface tension forms it into a spherical surface after which it is allowed to cool.

ln forming a convex, smooth, spherical surface in accordance with this portion of my invention the tip of the solid rod 3 formed in accordance with the methods illustrated in FIGS. 1 and 5 may be melted by electron beam bombardment and thereafter the spherical tip and a small portion of the rod cut from the remainder of the rod and mounted on refractory metal supports 6, 7. Alternatively, the cathode body 5 may be mounted in the supports prior to electron beam bombardment.

The formation of the spherical tip by surface tension in a molten state results in a very smooth, dense surface of accurate sphericity when a single solid phase or eutectic composition is used. In the immediate vicinity of this tip, the electric field is accurately radial and of high intensity. Electrons emitted from the surface tend to follow the field lines makirg them appear to come from the center of the sphere. Electron lenses, therefore, using such cathodes have a small virtual ob ject with which to form an image.

It is apparent from the foregoing that the techniques described may be applied to a variety of materials other than thorium carbide or mixtures of thorium carbide and tungsten. Thus, intermetallic compounds and, in particular, pure refractory metals which are also often used as cathodes in situations where the required emission current density is not excessively high, may be employed. By using the surface ,.;ergy of the liquid phase of the compound to cause the molten mixture rise in tube 3, I obtain a dense, compact cathode of uniform composition which does not shrink over long periods of heating to high temperatures but rather provides a long-life, uniform, emitting cathode body.

While in the foregoing I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

lower end is immersed in the molten mixture, allowing the mixture to ascend the tube by capillary attraction, and cooling the mixture in the tube.

2. The method of claim 1 which includes cutting the cooled tube with fused mixture therein into sections to form a plurality of cathode bodies of desired length.

3. The method of claim 1 wherein the mixture is melted in a refractory metal container.

4. The method of claim 1 wherein the refractory tube consists of tungsten and rhenium.

5. The method of claim 1 which includes the steps of suspending the refractory metal tube the above the powder mixture with a fusible support and heating the tube, mixture,

and support above the melting point of the support so that the tube drops into the molten mixture.

6. The method of claim 5 which includes providing guides for the dropping tube so that it descends substantially vertically into the mixture and is supported in the mixture.

7. The method of claim 6 which includes placing a weight at the top of the suspended tube to assist it in descending when the support fuses.

8. The method of claim 1 which includes the step of subjecting the end of a cathode body to a beam of electrons to melt the end and form The smooth, hemispherical surface. 

2. The method of claim 1 which includes cutting the cooled tube with fused mixture therein into sections to form a plurality of cathode bodies of desired length.
 3. The method of claim 1 wherein the mixture is melted in a refractory metal container.
 4. The method of claim 1 wherein the refractory tube consists of tungsten and rhenium.
 5. The method of claim 1 which includes the steps of suspending the refractory metal tube the above the powder mixture with a fusible support and heating the tube, mixture, and support above the melting point of the support so that the tube drops into the molten mixture.
 6. The method of claim 5 which includes providing guides for the dropping tube so that it descends substantially vertically into the mixture and is supported in the mixture.
 7. The method of claim 6 which includes placing a weight at the top of the suspended tube to assist it in descending when the support fuses.
 8. The method of claim 1 which includes the step of subjecting the end of a cathode body to a beam of electrons to melt the end and form The smooth, hemispherical surface. 